This invention relates in general to linear differential input amplifiers with a low differential input impedance, which is defined as that impedance between the differential input terminals, and a high common mode impedance, which is defined as that impedance between either input terminal and the common, ground, or reference terminal of an electrical network. Due to the high common mode impedance, such amplifiers can be used to measure the current or charge transfer through a branch of an electrical network without charging the impedances or voltages that would be observed if the amplifier were replaced by a floating shunt between the amplifier input nodes. In particular, the invention can be used as a direct current (DC) amplifier for the differential current or charge at the output of a two conductor transmission line attached to a circuit or transducer that has a balanced or unbalanced impedance or an unknown potential with respect to the common, ground, or reference terminal.
A short piece of insulated wire is one practical representative of an ideal floating shunt. In order to function as a floating shunt amplifier there must be some means of generating an output which is a function of the current through the wire. For example, the wire can be replaced by the primary winding of a coupling transformer, thereby achieving a relatively low differential input impedance together with a relatively high common mode impedance and reasonably good isolation between the common mode voltages at the primary and secondary sides of the transformer. However, certain disadvantages which will be later explained, are present when a transformer is utilized.
A particularly useful application of a floating shunt amplifier exists in the field of seismic surveying. Seismic surveys are generally conducted with remote transducer stations that are connected to a survey recording instrument via a seismic cable. A transducer station consists of either an individual transducer or of an array of transducers connected together with a coupling network. The transducers respond to sound waves by converting some aspect of acoustic wave propagation into electrical information. Typically this information is an output voltage which is proportional to the local acoustic pressure for hydrophone transducers, the local particle velocity for geophone transducers, or the local particle acceleration for accelerometer transducers. The seismic cable is used to conduct the electrical information from the transducer to the survey recording instrument and may include strain cables, structural spacers, mechanical connectors, protective jackets, transformers, coupling devices, equalization networks, electrical leads, and electrical connectors. In one commonly used form of seismic cable, some of the electrical leads are twisted pair transmission lines called cable pairs. Each transducer station is coupled through its own cable pair to an input amplifier stage of a particular channel of the survey recording instrument. The cable pair length can vary considerably from one transducer station to another. For example, on one marine survey vessel the cable pair length from the survey recording instrument to the nearest station is 775 feet, while that to the farthest staton is 12,695 feet. For a land seismic survey typical cable pair lengths may range from 80 feet to 19,200 feet.
Due to physical limitations, a transducer station does not behave like an ideal voltage source but manifests a finite source impedance. Likewise, each cable pair exhibits both series and shunt impedance distributed along the entire length of the transmission line. These impedance characteristics cause the electrical signal from the transducer station to undergo a frequency selective absorption and phase shift that varies with the length of the transmission line. Consequently, the electrical signal that is coupled to the input amplifier stage of the survey recording instrument is a distorted replica of the signal which is transmitted from the transducer station. Moreover, even if the transducer stations transmit identical signals, the input amplifier stages will receive an electrical signal which varies with the cable pair length between the transducer station and the survey recording instrument.
Some of the signal distortion can be alleviated by reducing either the source impedance or the load impedance attached to the transmission line. Transformers, for example, have been used with varying degrees of success to lower the source or the load impedance. However, when used to lower the source impedance, the transducer station output voltage is thereby reduced which can make the effect of Johnson noise or other noise voltages more serious. Even when used to lower the load impedance, transformers are not always desirable for use in a multiple channel seismic survey recording instrument for numerous reasons. For instance, even a well designed transformer will distort the signal at very low frequencies, the transformers in a multiple channel instrument may have to be oriented with respect to one another in order to minimize mutual coupling, magnetic transformers can be sensitive to external magnetic field pickup such as from power distribution lines, and transformers designed for the low frequencies used in seismic surveys tend to be heavy, bulky and expensive.
When the source impedance of the transducer station is capacitive such as for a crystal hydrophone transducer, another method of reducing the load impedance has been the use of differential charge amplifier, typified by U.S. Pat. No. 3,939,468 issued to Mastin. Two single-ended operational amplifier circuits each having identical parallel capacitive-resistive feedback loops are used to form a balanced differential charge amplifier stage which is coupled to a differential voltage amplifier stage. The circuit has a very low differential input impedance, thereby reducing the effects caused by the shunt impedance between the cable pair conductors. A high common mode rejection ratio is achieved by using the differential voltage amplifier to cancel the balanced common mode charge response from the differential charge amplifier stage.
The low differential input impedance of this amplifier reduces the effect of the total shunt impedance by maintaining the two input terminals at approximately the same potential, thereby allowing only a small differential voltage and hence very little current flow through the distributed shunt capacitance or other shunt impedances between the cable pair conductors. However, it does not compensate for the series impedance of the cable pair nor does it compensate for the residual effects of the distributed shunt impedance. Consequently, within the seismic survey frequency band, the output voltage of the charge amplifier circuit is proportional to the output of the transducer station only for moderately short transmission lines. The signal amplitude and phase responses change as the frequency and distance increase.
The variation in the amplitude and phase responses could be reduced by the addition of passive equalization networks to each cable pair, thus making the transfer function for each station approximately the same as that of the station with the longest transmission line. However, even after the transfer functions are equalized, subsequent filtering or digital data processing would be needed in order to obtain a flat, zero-phase shift response within a desired frequency passband.
Another difficulty with the Mastin circuit is that it has a very low common mode impedance between either input terminal and the common or ground terminal. As taught by Elio Poggiagliolmi (Slashes on Seismic Records, in IEEE Transactions on Geoscience Electronics, Vol. GE-15, No. 4, Oct. 1977, pp. 215-227.) each of the cable pairs in the seismic cable acts as an antenna for very low frequency (VLF) electromagnetic radiation such as that generated by thunder storms. This VLF antenna has a low radiation impedance with respect to the ground; consequently, the higher the common mode impedance the less energy is transferred into the amplifier. It follows that seismic instruments would be less sensitive to electromagnetic pickup if they could be isolated from the ground.
Another type of balanced charge amplifier circuit has been proposed by Hoffman, et al, in U.S. Pat. No. 3,469,255. An auxilliary circuit is used to effect a balanced sink for the common mode charge transfer so that the output of a single-ended type of charge amplifier can respond to only the differential charge transfer. The Hoffman circuit also exhibits a low differential input impedance and a low common mode impedance. Both the Mastin circuit and the Hoffman circuit shunt the common mode charge to ground through a very low impedance, hence both would be sensitive to VLF electromagnetic radiation as described by Poggiagliolmi. Their rejection of the common mode charge response depends upon equal impedances in both halves of the input circuit and of the feedback circuit.
In order to raise the common mode impedance of either of the above circuits, it is necessary to use additional circuitry. For example, a balance input isolation transformer will effect a high common mode impedance together with a low differential input impedance but it will also introduce the previously discussed disadvantages of coupling transformers. In some cases, it is practical to design an auxilliary circuit to make the voltage at the internal common terminal of the amplifier circuit equal to the external common mode voltage of the cable pair. Alternatively, the entire amplifier circuit including all power supplies can be insulated from the external common or ground. Both of these approaches have been used for certain types of instrumentation but they are not satisfactory solutions for a multiple channel seismic survey recording instrument because each amplifier circuit would have to be totally isolated from the common or ground terminal and from all of the other amplifier circuits. Consequently, a multitude of separate power supplies or external circuits would be required.